It is known in the art to provide light images by disposing a mask in front of output radiation from a laser. The principles involved are substantially the same as in normal light projectors used for projecting slides or motion picture film. In these devices, the entire output radiation impinges upon the mask and the transparent or cutout portions of the mask defining the pattern pass a percentage of the output radiation. While optical imaging of the pattern on the mask is possible with this system, it is very difficult to provide sufficient energy density or power in the image defined by the mask to effect operations other than that of simply imaging a picture; for example, drilling through a material or effecting a welding along a given pattern as defined by the mask.
The two major difficulties in attempting to perform such drilling or welding operations in accord with a pattern in the mask arises from the fact that, first sufficient energy or power is not available in the beam after it leaves the mask because of the eclipsing which cuts down a large part of the total power available and, second, if the overall power is increased sufficiently to attempt to solve this problem, the mask itself is often subject to destruction.
In simple light imaging systems, it has been proposed to incorporate a mask in the optical cavity of a laser so that the mask itself will not be subject to destruction. Such a system has been described in U.S. Pat. No. 3,293,565 issued to W. A. Hardy for a gas laser wherein curved end mirrors are required in order to effect the lasing action together with a suitable lens to project the image from the system. With such a gas laser, there is not nearly enough output energy in the projected beam to effect physical alternation of a target surface.